Making arsenic acid and arsenical compounds.



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GEORGE PRESCOTT FULLER, NLQrGARir FALLS, NEW YORK, ASSIGNORTO NATIONAL ELECTRGLYTIG COMPANY, OF NIAGARA FALLS, NEW T'Z'QRK, A CQRPOBAI'ION OF NEW YORK.

MAKING ABSENIC ACID AND ARSENICAL COMFOUTLJIFS.

icense...

No Drawing.

To all whom it may concern Be it known that I, Gnonon .lPansc-orr FULLER, a citizen of the United States, residing at'Niagara Falls, county of Niagara, and State of New York, have invented ccr tain new and useful Improvements in Male ing Arsenic .Acid and Arsenical Compounds, fully described and represented in the following specification.

This invention relates to the art of making arsenic acid and arsenical compounds.

The object of the invention. is to produce arsenic acid and arsenates in a simplcvand economical manner;

lVith this general object in View, the invention broadly considered consists in electrolyzing an aqueous solution of an alkali metal halid containing arsenious osid in suspension, whereby arsenic acid is produced, and then removing this arsenic acid from the solution, either as free arsenic acid or as an arsenate or other arsenical compound.

As a result of experiments, I have found that if arsenic trioxid (AS406) be suspended in an aqueous solution of an alkali metal halid and the solution then subjected to electrolysis, arsenic acid Will be found among the reaction products. I have also found that no undesirable secondary reacti ons occur in the process, such, for example, as would result in the deposition of arsenic on the cathode, or the generation of arsin,

I by cathodic reduction. Therefore, the elec- Scnic acid produced by the above process trolytic cell may conveniently be of the single compartment type as well as of the double compartment type, but, owing to the absence of undesirable secondary reactions as above pointed out, there is no advantage in the use of the more complicated form of cell in which a diaphragm is used. Furthermore, not necessary, to suitable amount of achromate or other add to the electrolyte a analogous substance which will prevent any cathodic reaction of hypochlorite. The anode must be insoluble and, for this purpose, I prcterably employ platinum. The cathode is best made of copper, although a ,cathode of other metals may be employed,

as will be obvious to those skilled in the art. As sodium arsenoteis the usua salt of arsenic acid employed in the a the er:

I have found it advantageous, though *vention is not Patented a... to, 1915.

Application filed .Tune 1. 191 Serial No. 842,024.

may be treated in solutionto convert it into such sodium arscnato, and then the latter re- 1 moved. In carrying out this process, it is of great advantage to use an electrolyte con sisting of an aqueous solution of a sodium halid, because the final removal of the ar- .senic acid as sodium arsenate is rendered less dillicult than it ursenates of two different alkali metals were present.

On account of it; cheapness, convenience, ready solubility, and other properties, I have found that sodium chlorid is aparticularly suitable alkali metal halid from which to prepare an aqueous solution to serve as the electrolyte. In making such an electrolyte, I find it best to employ the sodium chlorid in concentrations of about 100 to 300 grams per liter. Furthermore, the electrolysis is most advantageously conducted at somewhat elevated temperatures, 4C0 to 70 C. being "found most'favorable.

The arsenic trioxid As), used for carrying out my process is maintained in suspension in the idectrolyteby suitable agitation of the bath. The electrolyte containing the arscnimls oxid in suspension may be run through the cell continuously, the unoxidizec excess filtered out and the solution. concentrated, or the operation may be in 'tcrmittcnt. solid. oxid bcin added continrr ously to the electrolyte in the cell, and kept in suspension by suitable stirring. or other agitation of the bath. until any predetermined concentration of arsenic acid has been reached. This may vary according to the conditions of operation, the scale on' which the process is being u'orlwd and the cost of electricity and evapo ation. ordinary conditions, a concentration of from 25% to 40% would be satisfactory, although it is to be understood that my inlimited to this range of concentration. When the desired conccntru-o tion is reached. the liquid is Withdrawn from the celland the latter is recharged with fresh electrolyte.

In practice. I find it best to diwontinue the electrolysis before all ol the arsenic trioxid has been oxdizcd. and then remove the excess by filtration. This excess may be used in a succeeding operation. When the electrolyte is an aqueous solution of sodium chlorid, the filtered liquid product oil the electrolysis contains sodium chlorid and Under off). 1,12

arsenic acid with slight amounts of odium arsenate and hydrochloric acid. The arsenic acid may be recovered in the free state by evaporation of the liquid until the sodium chlorid separates out, whereupon the latter may be removed by filtering or most advantageously by centrifuging the liquid while hot. The sodium chlorid may be used in subsequent operations. After the removal of the sodium chlorid the solution may be allowed to cool, whereupon the arsenic acid will separate out if the solution was suiiiciently concentrated during the evaporation. The solution is again filtered to remove the arsenic acid, and the filtrate may be employed again in subsequent operations, if desired.

When the process is being worked for the production of suitable salts of arsenic acid, the filtered liquid product of electrolysis may be treated chemically to convert the arsenic acid into the desired salt, and the latter then removed by suitable operations. I have found it particularly advantageous to convert the arsenic acid into sodium. arsenate and then remove the latter from the solution. This may be done by reacting upon the filtered liquid product of electrolysis ,with sodium hydroxid, but T have found sodium carbonate to be most advantageous, because the salt thus obtained is the disodium arsenate, Na HASO which is the salt most used in the arts. When sodium hydroxid is employed, some trisodium arsenate, Na AsO will be obtained, unless the arsenic acid is in excess.

As one explanation of the results obtained by my improved process, the following is submitted as a statement of the reactions which probably take place when arsenic trioxid is subjected to electrolysis in an aqueous solution of sodium chlorid: The primary reaction will be the formation of sodium hypochlorite. The solution being in contact with solid arsenious oxid, the latter will. be dissolved until. the solution is saturated with respect to arsenious oxid and at this time contains about 3.5% of the latter. Thereafter, by a secondary and purely chemical reaction, the hypochlorite will be reduced by the arsenious oxid regenerating sodium chlorid and oxidizing to arsenic pentoxid. As rapidly as this reaction occurs,

' are probably the result of a double decomaces position occurring between small amounts of arsenic acid and sodium chlorid.

As a specific example of one way in which the process may be employed, the following; is submitted: An aqueous solution containing 30% of sodium chlorid may be used as .the electrolyte in a cell provided with a cop per cathode and a platinum anode. The electrolyte may be kept at a temperature of 56 G. and subjected to an E. M. of 3.1 volts. The arsenious oxid may-be introduced in intermittent charges with suitable agitation' to maintain it .in suspension in the elec trolyte. I have found that in carrying out this specific process and then removing the arsenic acid as an arsenate, 50 grams oil arsenic trioxid being charged at the com:- mencement and thereafter 25 grams lul minute intervals until 1500 grams in all had been added, there were obtained 2790 grains of disodium arsenate. A current eificiency of 96% was obtained.

ll hat is claimed is 1. The process which comprises electro lyzing a solution of an alkali metal halid in the presence of arsenious oxid.

2. The process which comprises suspending arsenious oxid in an aqueous solution of a halidof an alkalimetal and subjecting to electrolysis.

3. The process which comprises suspending arsenious oxid in an aqueous solution oi? sodium chlorid and subjecting to electrolysis.

4. The process which comprises suspending arsenious oxid in an aqueous solution of an alkali metal halid and subjecting to electrolysis, filtering the reaction product and treating the filtrate to recover the arsenical compounds therein. 7

5. The processof making arsenates, hich. comprises suspending arsenious oxid aqueous solution of an alkali metal. nalid,

subjecting to electrolysis, filtering the solution, neutralizing the filtrate to produce form an arsenate, and then recovering the arsenate of the alkali metal. 4

8. The process of making arsenates, which comprises maintaining arsenic trioxid in. suspension in an aqueous solution of sodium chlorid while subjecting the solution to the action of an electrolyzi ng current to produce arsenic acid, filtering the solution to remove unconverted arsenic trioxid, treat ing the filtrate with I sodium carbonate to 5 convert the arsenic acid'to disodium arsenate and separating out thelatter 9. The process which comprises electrolyzing a solution of an .alkali metal halid in the presence of arsenious oxid and material 10 capable of preventing cathodic reaction of hypochlorite.

10. The process whichfcomprises electrolyzinga solution of an alkali metal in the presence of arsenious oxid and a chromate which will prevent cathodic reaction of hypochlorite.

In testimony whereof, I havehereunto set my hand, in the presence of two silhscribing. Witnesses.

GEORGE PRESCOTT FULLER.

l/Vitnesses:

H. G. LAUBQCKER, V. J STAFFQRD. 

